Equal power output spatial spreading matrix for use in a wireless MIMO communication system

ABSTRACT

A transceiver utilizes a spatial spreading matrix to distribute two or more encoded spatial data streams to multiple antennas. The spatial spreading matrix satisfies one or more of the following two constraints: (a) the ratio of squared norms of the sum of the components of a row, for different rows of the spatial spreading matrix, is equal to a first constant sequence, and (b) the ratio of squared norms of the sum of a symbol S 1  to be transmitted, when the symbol S 1  is equal to 1 or −1, multiplied by each of the components of a row, for different rows of the spatial spreading matrix, is equal to a second constant sequence.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/932,862, entitled “Equal Power Output Spatial SpreadingMatrix for Use in a Wireless MIMO Communication System,” filed on Jul.1, 2013, now U.S. Pat. No. 8,767,863, which is a continuation of U.S.patent application Ser. No. 13/567,763, entitled “Equal Power OutputSpatial Spreading Matrix for use in a Wireless MIMO CommunicationSystem,” filed on Aug. 6, 2012, now U.S. Pat. No. 8,477,869, which is acontinuation of U.S. patent application Ser. No. 12/788,081, entitled“Equal Power Output Spatial Spreading Matrix for Use In a Wireless MIMOCommunication System,” filed on May 26, 2010, now U.S. Pat. No.8,238,466, which is a continuation of U.S. patent application Ser. No.11/851,237, entitled “Equal Power Output Spatial Spreading Matrix forUse In a Wireless MIMO Communication System,” filed on Sep. 6, 2007, nowU.S. Pat. No. 7,751,495, which claims priority to U.S. ProvisionalApplication No. 60/824,701, entitled “Spatial Spreading Matrix DesignFor Equal Power Output on All Transmit Antennas,” filed on Sep. 6, 2006.All of the applications referenced above are hereby incorporated byreference herein in their entireties.

FIELD OF TECHNOLOGY

The present disclosure relates generally to wireless communicationsystems and, more particularly, to a system and method for thesimultaneous transmission of multiple streams of information or data ina multiple-input, multiple-output wireless communication system.

BACKGROUND

An ever-increasing number of relatively cheap, low power wireless datacommunication services, networks and devices have been made availableover the past number of years, promising near wire speed transmissionand reliability. Various wireless technologies are described in detailin the 802.11 IEEE Standard, including for example, the IEEE Standard802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g(2003), as well as the IEEE Standard 802.11n now in the process of beingadopted, all of which are collectively incorporated herein fully byreference. These standards have been or are in the process of beingcommercialized with the promise of 54 Mbps or more effective throughput,making them a strong competitor to traditional wired Ethernet and themore ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmissionstandard.

Generally speaking, transmission systems compliant with the IEEE 802.11aand 802.11g or “802.11a/g” as well as the 802.11n standards achievetheir high data transmission rates using Orthogonal Frequency DivisionModulation or OFDM encoded symbols mapped up to a 64 quadratureamplitude modulation (QAM) multi-carrier constellation. In a generalsense, the use of OFDM divides the overall system bandwidth into anumber of frequency sub-bands or channels, with each frequency sub-bandbeing associated with a respective sub-carrier upon which data may bemodulated. Thus, each frequency sub-band of the OFDM system may beviewed as an independent transmission channel within which to send data,thereby increasing the overall throughput or transmission rate of thecommunication system.

Transmitters used in the wireless communication systems that arecompliant with the aforementioned 802.11a/802.11g/802.11n standards aswell as other standards such as the 802.16a IEEE Standard, typicallyperform multi-carrier OFDM symbol encoding (which may include errorcorrection encoding and interleaving), convert the encoded symbols intothe time domain using Inverse Fast Fourier Transform (IFFT) techniques,and perform digital to analog conversion and conventional radiofrequency (RF) upconversion on the signals. These transmitters thentransmit the modulated and upconverted signals after appropriate poweramplification to one or more receivers, resulting in a relativelyhigh-speed time domain signal with a large peak-to-average ratio (PAR).

Likewise, the receivers used in the wireless communication systems thatare compliant with the aforementioned 802.11a/802.11g/802.11n and802.16a IEEE standards typically include an RF receiving unit thatperforms RF downconversion and filtering of the received signals (whichmay be performed in one or more stages), and a baseband processor unitthat processes the OFDM encoded symbols bearing the data of interest.The digital form of each OFDM symbol presented in the frequency domainis recovered after baseband downconverting, conventional analog todigital conversion and Fast Fourier Transformation of the received timedomain signal. Thereafter, the baseband processor performs demodulationand frequency domain equalization (FEQ) to recover the transmittedsymbols, and these symbols are then processed with an appropriate FECdecoder, e.g. a Viterbi decoder, to estimate or determine the mostlikely identity of the transmitted symbol. The recovered and recognizedstream of symbols is then decoded, which may include deinterleaving anderror correction using any of a number of known error correctiontechniques, to produce a set of recovered signals corresponding to theoriginal signals transmitted by the transmitter.

In wireless communication systems, the RF modulated signals generated bythe transmitter may reach a particular receiver via a number ofdifferent propagation paths, the characteristics of which typicallychange over time due to the phenomena of multi-path and fading.Moreover, the characteristics of a propagation channel differ or varybased on the frequency of propagation. To compensate for the timevarying, frequency selective nature of the propagation effects, andgenerally to enhance effective encoding and modulation in a wirelesscommunication system, each receiver of the wireless communication systemmay periodically develop or collect channel state information (CSI) foreach of the frequency channels, such as the channels associated witheach of the OFDM sub-bands discussed above. Generally speaking, CSI isinformation describing one or more characteristics of each of the OFDMchannels (for example, the gain, the phase and the SNR of each channel).Upon determining the CSI for one or more channels, the receiver may sendthis CSI back to the transmitter, which may use the CSI for each channelto precondition the signals transmitted using that channel so as tocompensate for the varying propagation effects of each of the channels.

To further increase the number of signals which may be propagated in thecommunication system and/or to compensate for deleterious effectsassociated with the various propagation paths, and to thereby improvetransmission performance, it is known to use multiple transmission andreceive antennas within a wireless transmission system. Such a system iscommonly referred to as a multiple-input, multiple-output (MIMO)wireless transmission system and is specifically provided for within the802.11n IEEE Standard now being adopted. As is known, the use of MIMOtechnology produces significant increases in spectral efficiency,throughput and link reliability, and these benefits generally increaseas the number of transmission and receive antennas within the MIMOsystem increases.

In particular, in addition to the frequency channels created by the useof OFDM, a MIMO channel formed by the various transmission and receiveantennas between a particular transmitter and a particular receiverincludes a number of independent spatial channels. As is known, awireless MIMO communication system can provide improved performance(e.g., increased transmission capacity) by utilizing the additionaldimensionalities created by these spatial channels for the transmissionof additional data. Of course, the spatial channels of a wideband MIMOsystem may experience different channel conditions (e.g., differentfading and multi-path effects) across the overall system bandwidth andmay therefore achieve different SNRs at different frequencies (i.e., atthe different OFDM frequency sub-bands) of the overall system bandwidth.Consequently, the number of information bits per modulation symbol(i.e., the data rate) that may be transmitted using the differentfrequency sub-bands of each spatial channel for a particular level ofperformance may differ from frequency sub-band to frequency sub-band.

It is known that the use of multiple spatial channels in a MIMO systemsignificantly increases throughput of the system as multiple streams ofdata can be sent through the system simultaneously. Thus, the use ofmultiple antennas within the MIMO system allows the use of multiplespatial streams, each of which includes streams of encoded data that areindependently modulated and transmitted from the antennas. Generallyspeaking, the number of spatial streams is less than or is equal to thenumber of transmit antennas. When the number of transmit antennas isequal to the number of spatial streams, the modulated symbols of thespatial stream are spread evenly across the transmission antennas (i.e.,one spatial stream per antenna) and are transmitted in parallel from thetransmission antennas. However, when the number of spatial streams isless than number of transmission antennas, a spatial spreading matrix isused to map the spatial streams onto the transmission antennas toprovide for maximum usage of the transmission antennas and thus maximumthroughput. Generally speaking, it is possible to use a differentspatial spreading matrix for each of the separate or possible tones orcombinations of tones of the modulation system (wherein each tonerelates to a different one of the possible symbols) to thereby allocateor to provide a spatial spreading matrix for use with the system that isoptimally configured to send each of the separate tones. However, thissystem requires storing of a significant number of different spatialspreading matrices based on the tones, the number of tones and thecombinations of tones sent in the system, and thus requires asignificant amount of memory to store the spatial spreading matrixes.This requirement is especially true in the larger systems that have asignificant number of spatial streams and/or transmission antennas.Generally speaking, in these systems as well as in other systems, thespatial spreading matrix is chosen to have orthogonal columns, so as toallocate the same amount of energy in each spatial stream.

However, from an implementation perspective, it is easier to design atransmission system having a single spatial spreading matrix that isused for all of the possible tones or combinations of tones. In thepast, it was known and generally accepted to use a discrete Fouriertransform (DFT) unity matrix as the spatial spreading matrix when thenumber of spatial streams was the same as the number of transmissionantennas. Moreover, it has been typical to use only a portion of the DFTunity matrix (determined for the number of transmission antennas beingused) when the number of spatial streams is less than the number oftransmission antennas. Thus, in a system in which two spatial streamsare transmitted simultaneously through three transmission antennas, twocolumns of the three-by-three DFT unitary matrix might be used as thespatial spreading matrix.

While such a system is generally acceptable when the data being sentwithin the separate signal streams is uncorrelated, and thus is randomwith respect to one another, problems can arise when the data being sentbetween the separate signal streams is correlated, which frequentlyoccurs in communication systems that have predefined headers such as incommunication systems using the 802.11(x) standards. In this case,significant portions of the symbol bit streams within the two separatespatial streams are correlated with one another, and can result in oneof the transmission antennas transmitting a significantly higher powerthan the other transmission antennas. Thus, for example, when threetransmission antennas are used to transmit two separate bit or symbolstreams, and the two separate bit or symbol streams have the identicaldata, the first transmission antenna might end up being used to transmitat, for example, four times the power as the other two transmissionantennas.

This unequal power situation causes a problem because the poweramplifiers used in transmission systems generally have non-linearcharacteristics when operated well outside of a normal operating range.Thus, if two transmission antennas transmit at a particular power whichis within the normal operating range of the associated power amplifiers,and one of the transmission antennas transmits at four times that power,this last transmission antenna may operate in a non-linear or abnormalregion of the power amplifier, causing the power amplifier of this lasttransmission antenna to fail to properly amplify the signal as comparedto the amplification provided by the other two transmission paths. Thesenon-linearities result, on the receiver side of the transmission system,in distortions within the data, which leads to possible improperdecoding of symbols at the receiver side, resulting in high and possiblyunacceptable data error rates.

SUMMARY OF THE DISCLOSURE

In one embodiment, an apparatus comprises a transceiver configured to:produce two or more encoded spatial data streams, wherein a number ofthe encoded spatial data streams is different than a number of antennasto be used to transmit the encoded spatial data streams, and utilize aspatial spreading matrix to distribute the two or more encoded spatialdata streams to the antennas. Additionally, the transceiver includes amemory to store the spatial spreading matrix. The spatial spreadingmatrix has components (i) associated with each row of a row dimensionhaving a number of rows equal to the number of the antennas to be usedto transmit the encoded spatial data streams and (ii) associated witheach column of a column dimension having a number of columns equal tothe number of the encoded spatial data streams to be transmitted. Also,the spatial spreading matrix satisfies one or more of the following twoconstraints: (a) the ratio of squared norms of the sum of the componentsof a row, for different rows of the spatial spreading matrix, is equalto a first constant sequence, and (b) the ratio of squared norms of thesum of a symbol S1 to be transmitted, when the symbol S1 is equal to 1or −1, multiplied by each of the components of a row, for different rowsof the spatial spreading matrix, is equal to a second constant sequence.

In another embodiment, an apparatus comprises a transceiver configuredto: produce two or more encoded spatial data streams for transmissionvia a plurality of antennas, and utilize a spatial spreading matrix todistribute the two or more encoded spatial data streams to the antennas.Also, the transceiver includes a memory to store the spatial spreadingmatrix. The spatial spreading matrix has components (i) associated witheach row of a row dimension having a number of rows equal to the numberof the transmission antennas to be used to transmit the two or moreencoded spatial data streams and (ii) associated with each column of acolumn dimension having a number of columns equal to a number of theencoded spatial data streams to be transmitted. Additionally, the sum ofsquared norms of the components of a row is equal to the squared norm ofthe sum of the components of the row for each row of the spatialspreading matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless MIMO communication system thatdetermines and uses a spatial spreading matrix to provide for equalpower output at all of the transmission antennas of a transmitter; and

FIGS. 2A-2H illustrate examples of various different devices in which awireless communication system implementing the transmission techniquesdescribed herein may be used.

DETAILED DESCRIPTION

While the transmission techniques described herein for processing andeffecting a wireless data transmission are described as being used incommunication systems that use one of the IEEE Standard 802.11(x)communication standards, these techniques may be used in various othertypes of wireless communication systems and are not limited to thoseconforming to one or more of the IEEE Standard 802.11(x) standards.

Referring now to FIG. 1, a MIMO communication system 10 is illustratedin block diagram form as generally including a single transmitter 12having multiple transmission antennas 14A-14N and a single receiver 16having multiple receiver antennas 18A-18M. The number of transmissionantennas 14A-14N can be the same as, more than, or less than the numberof receiver antennas 18A-18M. As shown in FIG. 1, the transmitter 12 mayinclude a controller 20 coupled to a memory 21, to a symbol encoder andmodulator unit 22 and to a spatial spreading block 24. The transmitter12 may also include a matrix equalizer 25 and a symbol demodulator anddecoder unit 26 to perform demodulation and decoding of signals receivedvia the antennas 14A-14N in a receive mode. Additionally, in someinstances, the transmitter 12 may include a spreading matrix calculationunit 28.

The controller 12 may be any desired type of controller and thecontroller 12 and the spreading matrix calculation unit 28 may beimplemented as one or more standard multi-purpose, programmableprocessors, such as micro-processors, as application specific integratedcircuits (ASICs), etc., or may be implemented using any other desiredtypes of hardware, software and/or firmware. Likewise, the spatialspreading block 24, and the matrix equalizer 25 may be implemented usingknown or standard hardware and/or software elements. If desired, variousof the transmitter components, such as the controller 20, the modulatorunit 22, the demodulator unit 26, the spreading matrix calculation unit28, the spatial spreading block 24 and the matrix equalizer 25 may beimplemented in the same or in different hardware devices, such as in thesame or different processors. Additionally, each of these components ofthe transmitter 12 may be disposed in a housing 31 (shown in dottedrelief in FIG. 1) and the routines or instructions for implementing thefunctionality of any of these components may be stored in the memory 21or within other memory devices associated with the individual hardwareused to implement these components.

Likewise, if desired, one or more pre-calculated or predeterminedspreading matrices may be stored in the memory 21 and used in thespreading matrix block 24 at various times or under various differentconditions. Thus, for example, a different pre-calculated orpredetermined spatial spreading matrix may be stored for each of anumber of possible combinations of encoded spatial streams of data to betransmitted and transmission antennas 14 to be used to simultaneouslytransmit these encoded spatial streams of data. Thus, for example, adifferent spatial spreading matrix may be calculated and stored for twospatial streams of data being sent via three of the transmissionantennas 14, for two spatial streams of data being sent via four of thetransmission antennas 14, for three spatial streams of data being sentvia five transmission antennas 14, etc. In this manner, thecommunication system 10 may optimally send different numbers of spatialstreams of data at different times, depending on the load of the system.Moreover, the communication system 10 may also use these variousdifferent pre-stored or pre-calculated spatial spreading matrices toaccount for or to adapt to the loss of one or more of the transmissionantennas 14 to be used in sending data within the communication system10.

During operation, information signals T_(x1)-T_(xn) which are to betransmitted from the transmitter 12 to the receiver 16 are provided tothe symbol encoder and modulator unit 22 for encoding and modulation. Ofcourse, any desired number of signals T_(x1)-T_(xn) may be provided tothe modulator unit 22, with this number generally being limited by themodulation scheme used by and the bandwidth associated with the MIMOcommunication system 10. Additionally, the signals T_(x1)-T_(xn) may beany type of signals, including analog or digital signals, and mayrepresent any desired type of data or information. Additionally, ifdesired, a known test or control signal C_(x1) (which may be stored inthe memory 21) may be provided to the symbol encoder and modulator unit22 for use in determining CSI related information describing thecharacteristics of the channel(s) between the transmitter 12 and thereceiver 16. If desired, the same control signal or a different controlsignal may be used to determine the CSI for each frequency and/orspatial channel used in the MIMO communication system 10.

The symbol encoder and modulator unit 22 may interleave digitalrepresentations of the various signals T_(x1)-T_(xn) and C_(x1) and mayperform any other known type(s) of error-correction encoding on thesignals T_(x1)-T_(xn) and C_(x1) to produce one or more encoded streamsof symbols SS₁, SS₂, . . . SS_(p), to be modulated and sent from thetransmitter 12 to the receiver 16. While the symbol streams SS₁-SS_(p)may be modulated using any desired or suitable QAM technique, such asusing 64 QAM, these symbols may be modulated in any other known ordesired manner including, for example, using any other desired phaseand/or frequency modulation techniques. In any event, the modulatedencoded symbol streams SS₁-SS_(p) are provided by the symbol encoder andmodulator unit 22 to the spatial spreading block 24 for processingbefore being transmitted via the antennas 14A-14N. While notspecifically shown in FIG. 1, the modulated symbol streams SS₁-SS_(p)may be processed by the spatial spreading block 24 that implements aspatial spreading matrix in accordance with a transmission techniquemore specifically described below, before being up-converted to the RFcarrier frequencies associated with an OFDM technique (in one or morestages). Upon receiving the modulated signals, the spatial spreadingblock 24 processes the modulated signals by injecting delays and/orgains into the modulated signals based on a spatial spreading matrixwhich may be provided by, for example, the controller 12, to therebyperform mixing and transmission of the spatial streams of data acrossthe transmission antennas 14A-14N.

The signals transmitted by the transmitter 12 are detected by thereceiver antennas 18A-18M and may be processed by a matrix equalizer 35within the receiver 16 to enhance the reception capabilities of theantennas 18A-18M. As will be understood, the processing applied at thereceiver 16 (as well as at the transmitter 12) may be based on, forexample, the CSI developed by the receiver 16 in response to thetransmission of the test or control signal C_(x1). In particular, acontroller 40 or other unit within the receiver 16, such as a channeldetermination unit 39, may process the received control signal C_(x1)and develop therefrom a measured description of the forward channelbetween the transmitter 12 and the receiver 16 by determining orcharacterizing the propagation effects of the forward channel on thesignal C_(x1) as it traveled through the forward channel. In any event,a symbol demodulator and decoder unit 36, under control of thecontroller 40, may decode and demodulate the received symbol stringsSS₁-SS_(p) as recovered by the matrix equalizer 35. In this process,these signals may be downconverted to baseband. Generally, thedemodulator and decoder unit 36 may operate to remove effects of theforward channel based on the CSI as well as to perform demodulation onthe received symbols in each symbol stream SS₁-SS_(p) to produce adigital bit stream for each stream. In some cases, if desired, thesymbol demodulator and decoder unit 36 may perform error correctiondecoding and deinterleaving on the bit stream to produce the receivedsignals R_(x1)-R_(xn) corresponding to the originally transmittedsignals T_(x1)-T_(xn).

As shown in FIG. 1, the receiver 16 may also include a memory 41 and asymbol encoder and modulator unit 46 which may receive one or moresignals T_(R1)-T_(Rm) encoded and modulated using any desired encodingand modulation techniques. The receiver 16 may also provide one or moreknown test or control signals C_(R1) to the symbol encoder/modulatorunit 46 to be sent to the transmitter 12 to enable the transmitter 12 todetermine a measured description of the reverse channel between thereceiver 16 and the transmitter 12. The encoded and modulated symbolstream may then be upconverted and processed by a spatial spreadingblock 34 which may use a spatial spreading matrix developed according tothe principles described herein based on the number of symbol streams tobe sent simultaneously and the number of transmission antennas 18 to beused. The output of the spatial spreading block 34 is then transmittedvia the receiver antennas 18A-18N to, for example, the transmitter 12,thereby implementing the reverse link. As shown in FIG. 1, each of thereceiver components may be disposed in a housing 51.

The matrix equalizer 25 and the demodulator/decoder unit 26 within thetransmitter 12 operate similarly to the matrix equalizer 35 and thedemodulator/decoder unit 36 of the receiver 16 to demodulate and decodethe symbol streams transmitted by the receiver 16 to produce therecovered signals R_(R1)-R_(Rm). Here again, the matrix equalizer 25 mayprocess the received signals in any known manner to enhance theseparation and therefore the reception of the various symbol streamstransmitted by the antennas 18A-18M. Of course, the CSI or othermeasured description of the forward channel for the various OFDMchannel(s) may be used to process or decode the received signals.

As indicated above, it is desirable to use a spatial spreading matrixwhich produces a power output proportional to the linear operatingregions of the power amplifiers for each of the transmission antennasunder all conditions, including when the data within the separate symbolstreams SS₁-SS_(p) is either correlated or uncorrelated. Typically, anymatrix with orthogonal columns is used as a spreading matrix, becausesuch a matrix allocates the same energy to each steam. Generallyspeaking, much of the information or data within the data packets of thestreams of data being sent using, for example the 802.11n standard, isuncorrelated, as the data in one symbol stream it is not related to datain other symbols steams, in which case these spatial spreading matriceswork as intended. However, there are certain portions of the datapackets used in the 802.11n standard which may contain highly correlateddata across spatial streams, including packet header information withinthe data packets of the various different symbol streams. For example,the HT-SIG and the HT-LTF portion of the data packets of the 802.11nstandard results in spatial streams that are not uncorrelated on aper-OFDM symbol basis. In fact, during the HT-SIG section, all spatialstreams are identical while, during a particular HT-LTF section, any twospatial symbols are either identical or are negatives of one another.These situations result in highly correlated data within the variousspatial data streams being transmitted which can result, using manyprior art transmission techniques, in a condition in which one of thetransmission antennas used to transmit the separate data streamsoperates at a significantly different power output level than other onesof the antennas, thereby potentially leading to transmission anddecoding problems.

A spatial spreading matrix that provides for power output that isproportional to the operating regions of the corresponding poweramplifiers across all of the transmission antennas of a transmissionsystem when transmitting correlated and uncorrelated data within two ormore spatial streams of data being sent simultaneously via two or moretransmission antennas may be determined as a spatial spreading matrixthat satisfies five separate constraints, as explained in more detailbelow. In particular, when the spatial spreading matrix is not a squarematrix because the number of transmission antennas is greater than thenumber of spatial streams, the spatial spreading matrix will stillproduce equal power output across the antennas in all cases when it isconfigured or determined so as to satisfy a combination of and,preferably, all five of separate constraints discussed below. Thus, anyspatial spreading matrix which satisfies all of these constraints (or insome cases some combination of a sub-set of these constraints) can beused in the spatial spreading block 24 of FIG. 1 to deliver equal orappropriately proportional power output across the separate transmissionantennas 14 in all conditions, that is when the data or symbols beingsent are either correlated or uncorrelated.

In particular, the goal is to design a spatial spreading matrix Q whichis the same for all tones k so that the spatial spreading matrix foreach tone Q_(k)=Q. The five constraints which are defined belowgenerally in an equation format may be used to design or calculate aspatial spreading matrix which reaches this goal. In particular, whenthe data streams are uncorrelated, the transmit power of the i^(th)antenna during the data segment of a packet is proportional to thesquare of the i^(th) row norm of the spatial spreading matrix Q becausethe data streams are independent. If the power amplifiers for all of thetransmitters have similar performance behavior, the spatial spreadingmatrix Q should have equal row norms for all of the number oftransmitter N_(TX) rows of the matrix Q. Otherwise, the output power ofsome transmission antennas may be significantly higher than the others,leading to high distortion due to inherent non-linear input-outputcharacteristics of typical power amplifiers. This requirement leads tothe first three constraints defined below.

The first constraint, as defined in equation (1) below, basicallyrequires that the sum of the squared norms of the column components of acolumn of the spatial spreading matrix be the same for all of thespatial streams, i.e., for each column of the spatial spreading matrix.This constraint may be defined mathematically as:

$\begin{matrix}{{{\sum\limits_{t = 1}^{N_{TX}}\;{{Q\left( {t,l} \right)}}^{2}} = C},\;{{{for}\mspace{14mu}{each}\mspace{14mu}{and}\mspace{14mu}{every}\mspace{14mu}{spatial}\mspace{14mu}{stream}\mspace{11mu} l} - {1\mspace{14mu}\ldots\mspace{20mu} N_{SS}}}} & (1)\end{matrix}$wherein:

N_(TX)=number of transmitters;

N_(SS)=number of spatial streams;

C=a constant; and

Q(t,l)=the spatial spreading matrix component at row t, column l.

The second constraint, as defined in equation (2) below, requires thatthe inner product of any two different columns of the spatial spreadingmatrix be equal to zero for every set of two different columns (i.e.,for every set of two columns which are not the same column). Generallyspeaking, this constraint means that the columns of the spatialspreading matrix define orthogonal vectors with respect to each other,thereby leading to a zero cross product between any two differentcolumns. This constraint can be defined mathematically as:

$\begin{matrix}{{{\sum\limits_{t = 1}^{N_{TX}}\;{{Q\left( {t,l} \right)}{Q^{*}\left( {t,m} \right)}}} = 0},\mspace{14mu}{{{for}\mspace{14mu}{all}\mspace{14mu} l}\bumpeq\not{}m}} & (2)\end{matrix}$wherein:

N_(TX)=number of transmitters;

-   -   Q(t,l)=the spatial spreading matrix component at row t, column        l;        and    -   Q*(t,m)=the component of the transpose of the spatial spreading        matrix at row t, column m.

The third constraint, as defined in equation (3) below, requires thatthe sum of the squared norms of the row components of a row of thespatial spreading matrix is equal to a constant, and is the same foreach of the rows of the spatial spreading matrix, when all the transmitantennas have power amplifiers with similar operating behavior. Theconstant, defined in equation (3) below as “A,” need not be zero but caninstead be non-zero, although this constant should be the same for everyrow. This third constraint can be defined mathematically as:

$\begin{matrix}{{{\sum\limits_{l = 1}^{N_{SS}}\;{{Q\left( {t,l} \right)}}^{2}} = A},\mspace{14mu}{{{and}\mspace{14mu}{is}\mspace{14mu}{same}\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu} t} = {1\mspace{14mu}\ldots\mspace{14mu}{N_{TX}.}}}} & (3)\end{matrix}$wherein:

N_(SS)=number of spatial streams;

N_(TX)=number of transmitters;

Q(t,l)=the spatial spreading matrix component at row t, column l;

and

A=a constant.

If the transmit antennas have power amplifiers that have differentlinear operating regions, then the ratio of sum of the squared norms ofthe row components of a row, for different rows of the spatial spreadingmatrix, is the same as a constant sequence designed based on the linearoperating regions of the power amplifiers, denoted here by A₁, A₂, . . ., A_(NTX). This constraint is defined mathematically as:

$\begin{matrix}{{{\sum\limits_{l = 1}^{N_{SS}}\;{{Q\left( {t,l} \right)}}^{2}} = {A_{t}B}},\mspace{14mu}{{{for}\mspace{14mu}{all}\mspace{14mu} t} = {1\mspace{14mu}\ldots\mspace{14mu}{N_{TX}.}}}} & \left( {3a} \right)\end{matrix}$wherein:

N_(SS) number of spatial streams;

N_(TX)=number of transmitters;

Q(t,l)=the spatial spreading matrix component at row t, column l;

and

B=a constant.

Generally speaking, in the past, non-square spatial spreading matriceswhich were based on a unitary matrix (e.g., which were simply portionsof a DFT matrix), satisfied these three constraints. Furthermore, aspatial spreading matrix that satisfies these three constraints worksadequately to transmit different symbol streams when the data in thesymbol streams is uncorrelated.

However, the following two constraints are useful when spatial streamshaving correlated data are sent simultaneously via the same transmissionsystem, which is frequently the case with, for example, the HT-SIG andHT-LTF portions of data packets sent using the 802.11n standard. Thefourth constraint as defined in equation (4) below requires that thesquared norm of the sum of the row components of a row of the spatialspreading matrix is equal to a constant for each row of the spatialspreading matrix, and in particular, is equal to the same constant asthe sum of the squared norms of the row components defined by equation(3), when all the transmit antennas have power amplifiers with similaroperating behavior. This fourth constraint is related to the thirdconstraint in that it requires that, for a particular row, the sum ofthe squared norms for that row is equal to the squared norm of the sumof the components of that row. The fourth constraint can be definedmathematically as:

$\begin{matrix}{{{{\sum\limits_{l = 1}^{N_{SS}}\;{Q\left( {t,l} \right)}}}^{2} = A},\mspace{14mu}{{{and}\mspace{14mu}{is}\mspace{14mu}{same}\mspace{14mu}{for}\mspace{20mu}{all}\mspace{11mu} t} = {1\mspace{14mu}\ldots\mspace{14mu}{N_{TX}.}}}} & (4)\end{matrix}$wherein:

N_(SS)=number of spatial streams;

Q(t,l)=the spatial spreading matrix component at row t, column l;

and

A=a constant.

If the transmit antennas have power amplifiers that have differentlinear operating regions, then the ratio of squared norm of the sum ofthe row components of a row, for different rows of the spatial spreadingmatrix, is the same as the constant sequence designed based on thelinear operating regions of the power amplifiers: {A₁, A₂, . . . ,A_(NTX)}. This constraint is defined mathematically as:

$\begin{matrix}{{{\sum\limits_{l = 1}^{N_{SS}}\;{Q\left( {t,l} \right)}}}^{2} = {{A_{t}B\mspace{20mu}{for}\mspace{14mu}{all}\mspace{11mu} t} = {1\mspace{14mu}\ldots\mspace{14mu}{N_{TX}.}}}} & \left( {4a} \right)\end{matrix}$wherein:

N_(SS) number of spatial streams;

N_(TX)=number of transmitters;

Q(t,l)=the spatial spreading matrix component at row t, column l;

and

B=a constant.

The fifth constraint requires that the power within a particular rowwhen the symbol being transmitted is a 1 or a −1 should also be equal tothe constant A defined within the third and fourth constraints providedabove. Thus, the squared norm of the sum of the symbol S_(l) (when thesymbol S_(l) is equal to 1 or −1) multiplied by each o the rowcomponents of a row should also be equal to the constant A for each rowof the spatial spreading matrix, when all the transmit antennas havepower amplifiers with similar operating behavior. The fifth constraintcan be defined mathematically as:

$\begin{matrix}{{{{\sum\limits_{l = 1}^{N_{SS}}\;{S_{l\;}{Q\left( {t,l} \right)}}}}^{2} = A}\;} & (5)\end{matrix}$wherein:

N_(SS)=number of spatial streams;

Q(t,l)=the spatial spreading matrix component at row t, column l;

A=a constant; and

1 or −1 depending on the HT-LTF.

If the transmit antennas have power amplifiers that have differentlinear operating regions, then the ratio of squared norm of the sum ofthe symbol S_(l) (when the symbol S_(l) is equal to 1 or −1) multipliedby each of the row components of a row, for different rows of thespatial spreading matrix, is the same as the constant sequence designedbased on the linear operating regions of the power amplifiers: {A₁, A₂,. . . , A_(NTX)}. This constraint is defined mathematically as:

$\begin{matrix}{{{\sum\limits_{l = 1}^{N_{SS}}\;{S_{l\;}{Q\left( {t,l} \right)}}}}^{2} = {{A_{t}B\mspace{14mu}{for}\mspace{14mu}{all}\mspace{11mu} t} = {1\mspace{14mu}\ldots\mspace{14mu}{N_{TX}.}}}} & \left( {5a} \right)\end{matrix}$wherein:

N_(SS)=number of spatial streams;

N_(TX)=number of transmitters;

Q(t,l)=the spatial spreading matrix component at row t, column l;

and

B=a constant.

Any spatial spreading matrix which satisfies these five constraints (orin some cases, some combination of a subset of these constraints) can beused in any square transmission system (having the same number ofspatial streams and transmission antennas) or in any non-squaretransmission system (having multiple symbol streams of data transmittedvia a different number of transmission antennas) to assure equal poweroutput for each of the transmission antennas, averaged over each symbolperiod. However, the use of a spatial spreading matrix that satisfiesthese constraints is generally more relevant or useful in cases in whicha strict unity DFT matrix cannot be used because of the non-squarenature of the transmission system. In particular, a spatial spreadingmatrix as defined above can be used advantageously in non-square systemsand is especially advantageous when the number of transmission antennasis greater than the number of symbol streams and in which the number ofsymbol streams is greater than or equal to two. Some example spatialspreading matrices which satisfy the five constraints defined above areprovided below for a 3×2 system (three transmission antennas used tosimultaneously send two spatial streams), a 4×2 system (fourtransmission antennas used to simultaneously transmit two spatialstreams), and a 4×3 system (four transmission antennas used tosimultaneously transmit three spatial streams), all with the poweramplifiers for different transmit antennas having identical or similaroperating behavior.

$3 \times 2{\text{:}\begin{bmatrix}\sqrt{\frac{1}{3}} & {j\sqrt{\frac{1}{3}}} \\{\sqrt{\frac{2}{3}}\;{\cos\left( {15{^\circ}} \right)}} & {{- j}\sqrt{\frac{2}{3}}{\sin\left( {15{^\circ}} \right)}} \\{\sqrt{\frac{2}{3}}{\sin\left( {15{^\circ}} \right)}} & {{- j}\sqrt{\frac{2}{3}}{\cos\left( {15{^\circ}} \right)}}\end{bmatrix}}$

$4 \times 2{\text{:}\begin{bmatrix}{1/2} & {j/2} \\{1/2} & {j/2} \\{1/2} & {{- j}/2} \\{1/2} & {{- j}/2}\end{bmatrix}}$

$4 \times 3{\text{:}\begin{bmatrix}{1/\sqrt{2}} & {j/2} & 0 \\0 & {1/2} & {j/\sqrt{2}} \\{1/\sqrt{2}} & {{- j}/2} & 0 \\0 & {{- 1}/2} & {j/\sqrt{2}}\end{bmatrix}}$An example of spatial spreading matrix design based of the aboveconstraints for a 3×2 system, with the power amplifier for the firsttransmit antenna having a linear operating region that is roughly twotimes the operating region of the other transmit antennas, is shownbelow:

$3 \times 2{\text{:}\begin{bmatrix}\sqrt{\frac{1}{2}} & {j\sqrt{\frac{1}{2}}} \\\frac{1}{2} & {{- j}\frac{1}{2}} \\\frac{1}{2} & {{- j}\frac{1}{2}}\end{bmatrix}}$

However, it is noted that these spatial spreading matrices are notunique for these types of systems, and in fact other and differentspatial spreading matrices can be determined for 3×2 systems, 4×2systems and 4×3 systems which satisfy the five constraints definedherein. Still further, if desired, other spatial spreading matrices maybe determined and used for other system sizes or configurations, such as5×2, 5×3, 5×4, 6×2, 6×3, 6×4, etc. systems. As will be understood, themanner in which the spatial spreading matrices are calculated ordetermined is not particularly important, as long as the resultingspatial spreading matrices satisfy one or more, and preferably all ofthe five constraints defined above. Thus, any mathematical, heuristic,iterative, or trial and error method can be used to actually determineor compute any particular size of spatial spreading matrix thatsatisfies the constraints defined herein, and the method used to computea spatial spreading matrix that satisfies these constraints (or somecombination of these constraints) is not determinative. Moreover, thespecific orientation or definition of the “row” and “column” dimensionsof a matrix as used herein is arbitrary and is simply based onconvention, and therefore may be changed. Thus, for example, the rowdimension can be a horizontal dimension with the column dimension beinga vertical dimension (as assumed herein), or the row dimension can be avertical dimension with the column dimension being a horizontaldimension.

If desired, a spatial spreading matrix determined according to theprinciples defined above may be pre-calculated and stored in the memory21 of the transmitter 12 and may be used within the spatial spreadingblock 24 when needed. Still further, as noted above, a number ofdifferent spatial spreading matrices may be stored in the memory 21 ofthe transmitter 12 to be used in different situations, such as whendifferent numbers of encoded spatial streams of data are to betransmitted simultaneously or when different numbers of the transmissionantennas 14 are available. Thus, it may be possible to detect thefailure of one or more of the antennas 14 and still operate usingmultiple spatial streams by switching to the use of a new spatialspreading matrix configured to transmit the same number of spatialstreams using a fewer number of antennas. Likewise, it may be possibleto switch the number of spatial streams being sent via the same numberof transmission antennas 14 or to switch both the number of spatialstreams being sent and the number of transmission antennas 14 used bysimply obtaining from the memory 21 a new spatial spreading matrixdesigned for the particular combination of the number of spatial streamsto be sent and the number of available antennas. In other words, thetransmission system may switch between different modes, wherein eachmode as a unique combination of number of spatial streams and number ofantennas, and a separate or different spatial spreading matrix may bepre-calculated and stored to be used in the spatial spreading block 24for each of these different modes. However, instead of or in addition tostoring pre-determined spatial spreading matrices, one or more spatialspreading matrixes may be calculated during operation of the system by,for example, the spatial spreading matrix calculation unit 28.

Moreover, it will be understood that the actual spatial spreading matrixequations, e.g., the computation of a particular spatial spreadingmatrix that satisfies the constraints defined above, may be performed atany desired location within the wireless communication system 10 of FIG.1, including within the controller 20 or other hardware, software, orfirmware of the transmitter 12, as well as within the controller 40 orother hardware, software, or firmware of the receiver 16. Alternatively,the spatial spreading matrix may be pre-computed and stored in thememory 21 (or 41) or other memory of the system 10 prior to thetransmission system being used. The spatial spreading matrix or matricesmay also be computed or determine by a different device and may be sentto the transmitter 12 or the receiver 14 of the transmission system 10at any desired time.

Of course, the spatial spreading matrix technique described herein isnot limited to being used in a transmitter of a MIMO communicationsystem communicating with a single receiver of the MIMO communicationsystem, but can additionally be applied when a transmitter of a MIMOcommunication system is communicating with multiple receivers, each ofwhich has one or more receiver antennas associated therewith.

While the spatial spreading matrix calculations described herein aredescribed in one example as being implemented in software stored in, forexample, one of the memories 21, 41 and implemented on a processorassociated with one of the controllers 20, 40, or with the spatialspreading matrix calculation unit 28 of the MIMO communication system 10of FIG. 1, these routines may alternatively or additionally beimplemented in digital or analog hardware, firmware, applicationspecific integrated circuits, etc., as desired. If implemented insoftware, the routines may be stored in any computer readable memorysuch as in RAM, ROM, flash memory, a magnetic disk, a laser disk, orother storage medium. Likewise, this software may be delivered to a MIMOsystem device (such as a transmitter or a receiver) via any known ordesired delivery method including, for example, over a communicationchannel such as a telephone line, the Internet, a wireless connection,etc., or via a transportable medium, such as a computer-readable disk,flash drive, etc.

The present invention may be embodied in any type of wirelesscommunication system including, for example, ones used in wirelesscomputer systems such as those implemented via a local area network or awide area network, internet, cable and satellite based communicationsystems (such as internet, data, video and voice communication systems),wireless telephone systems (including cellular phone systems, voice overinternet protocol (VoIP) systems, home-based wireless telephone systems,etc.) Referring now to FIGS. 2A-2H, various exemplary implementations ofthe present invention are shown.

Referring to FIG. 2A, the present invention may be used with a hard diskdrive 400 which includes both signal processing and/or control circuits,which are generally identified in FIG. 2A at 402. In someimplementations, signal processing and/or control circuit 402 and/orother circuits (not shown) in HDD 400 may process data, perform codingand/or encryption, perform calculations, and/or format data that isoutput to and/or received from a magnetic storage medium 406.

HDD 400 may communicate with a host device (not shown) such as acomputer, mobile computing devices such as personal digital assistants,cellular phones, media or MP3 players and the like, and/or other devicesvia one or more wired or wireless communication links 408 which mayimplement the beamforming techniques described above. HDD 400 may beconnected to memory 409, such as a random access memory (RAM), a lowlatency nonvolatile memory such as flash memory, read only memory (ROM)and/or other suitable electronic data storage.

Referring now to FIG. 2B, the present invention may be embodied in orused with a digital versatile disc (DVD) drive 410 which may includeeither or both signal processing and/or control circuits, which aregenerally identified in FIG. 2B at 412, and/or mass data storage 418 ofDVD drive 410. Signal processing and/or control circuit 412 and/or othercircuits (not shown) in DVD 410 may process data, perform coding and/orencryption, perform calculations, and/or format data that is read fromand/or data written to an optical storage medium 416. In someimplementations, signal processing and/or control circuit 412 and/orother circuits (not shown) in DVD 410 can also perform other functionssuch as encoding and/or decoding and/or any other signal processingfunctions associated with a DVD drive.

DVD drive 410 may communicate with an output device (not shown) such asa computer, television or other device via one or more wired or wirelesscommunication links 417 which may be implemented using the beamformingtechniques described above. DVD 410 may communicate with mass datastorage 418 that stores data in a nonvolatile manner. Mass data storage418 may include a hard disk drive (HDD) such as that shown in FIG. 2A.The HDD may be a mini HDD that includes one or more platters having adiameter that is smaller than approximately 1.8″. DVD 410 may beconnected to memory 419, such as RAM, ROM, low latency nonvolatilememory such as flash memory, and/or other suitable electronic datastorage.

Referring now to FIG. 2C, the present invention may be embodied in ahigh definition television (HDTV) 420 which may include either or bothsignal processing and/or control circuits, which are generallyidentified in FIG. 2C at 422, a WLAN interface and/or mass data storageof the HDTV 420. HDTV 420 receives HDTV input signals in either a wiredor wireless format and generates HDTV output signals for a display 426.In some implementations, signal processing circuit and/or controlcircuit 422 and/or other circuits (not shown) of HDTV 420 may processdata, perform coding and/or encryption, perform calculations, formatdata and/or perform any other type of HDTV processing that may berequired.

HDTV 420 may communicate with mass data storage 427 that stores data ina nonvolatile manner such as optical and/or magnetic storage devices. Atleast one HDD may have the configuration shown in FIG. 2A and/or atleast one DVD may have the configuration shown in FIG. 2B. The HDD maybe a mini HDD that includes one or more platters having a diameter thatis smaller than approximately 1.8″. HDTV 420 may be connected to memory428 such as RAM, ROM, low latency nonvolatile memory such as flashmemory and/or other suitable electronic data storage. HDTV 420 also maysupport connections with a WLAN via a WLAN network interface 429 whichmay implement the beamforming techniques described above.

Referring now to FIG. 2D, the present invention may be used inconjunction with a control system of a vehicle 430 having a WLANinterface and/or mass data storage. In some implementations, the presentinvention may be used within a powertrain control system 432 thatreceives inputs from one or more sensors such as temperature sensors,pressure sensors, rotational sensors, airflow sensors and/or any othersuitable sensors and/or that generates one or more output controlsignals such as engine operating parameters, transmission operatingparameters, and/or other control signals.

The present invention may also be embodied in other control systems 440of vehicle 430. Control system 440 may likewise receive signals frominput sensors 442 and/or output control signals to one or more outputdevices 444. In some implementations, control system 440 may be part ofan anti-lock braking system (ABS), a navigation system, a telematicssystem, a vehicle telematics system, a lane departure system, anadaptive cruise control system, a vehicle entertainment system such as astereo, DVD, compact disc and the like. Still other implementations arecontemplated.

Powertrain control system 432 may communicate with mass data storage 446that stores data in a nonvolatile manner. Mass data storage 446 mayinclude optical and/or magnetic storage devices for example hard diskdrives HDD and/or DVDs. At least one HDD may have the configurationshown in FIG. 2A and/or at least one DVD may have the configurationshown in FIG. 2B. The HDD may be a mini HDD that includes one or moreplatters having a diameter that is smaller than approximately 1.8″.Powertrain control system 432 may be connected to memory 447 such asRAM, ROM, low latency nonvolatile memory such as flash memory and/orother suitable electronic data storage. Powertrain control system 432also may support connections with a WLAN via a WLAN network interface448 which may implement the beamforming techniques described above. Thecontrol system 440 may also include mass data storage, memory and/or aWLAN interface (all not shown).

Referring now to FIG. 2E, the present invention may be embodied in acellular phone 450 that may include one or more cellular antennas 451,either or both signal processing and/or control circuits, which aregenerally identified in FIG. 2E at 452, a WLAN interface and/or massdata storage of the cellular phone 450. In some implementations,cellular phone 450 includes a microphone 456, an audio output 458 suchas a speaker and/or audio output jack, a display 460 and/or an inputdevice 462 such as a keypad, pointing device, voice actuation and/orother input device. Signal processing and/or control circuits 452 and/orother circuits (not shown) in cellular phone 450 may process data,perform coding and/or encryption, perform calculations, format dataand/or perform other cellular phone functions.

Cellular phone 450 may communicate with mass data storage 464 thatstores data in a nonvolatile manner such as optical and/or magneticstorage devices for example hard disk drives HDD and/or DVDs. At leastone HDD may have the configuration shown in FIG. 2A and/or at least oneDVD may have the configuration shown in FIG. 2B. The HDD may be a miniHDD that includes one or more platters having a diameter that is smallerthan approximately 1.8″. Cellular phone 450 may be connected to memory466 such as RAM, ROM, low latency nonvolatile memory such as flashmemory and/or other suitable electronic data storage. Cellular phone 450also may support connections with a WLAN via a WLAN network interface468.

Referring now to FIG. 2F, the present invention may be embodied in a settop box 480 including either or both signal processing and/or controlcircuits, which are generally identified in FIG. 2F at 484, a WLANinterface and/or mass data storage of the set top box 480. Set top box480 receives signals from a source such as a broadband source andoutputs standard and/or high definition audio/video signals suitable fora display 488 such as a television and/or monitor and/or other videoand/or audio output devices. Signal processing and/or control circuits484 and/or other circuits (not shown) of the set top box 480 may processdata, perform coding and/or encryption, perform calculations, formatdata and/or perform any other set top box function.

Set top box 480 may communicate with mass data storage 490 that storesdata in a nonvolatile manner. Mass data storage 490 may include opticaland/or magnetic storage devices for example hard disk drives HDD and/orDVDs. At least one HDD may have the configuration shown in FIG. 2Aand/or at least one DVD may have the configuration shown in FIG. 2B. TheHDD may be a mini HDD that includes one or more platters having adiameter that is smaller than approximately 1.8″. Set top box 480 may beconnected to memory 494 such as RAM, ROM, low latency nonvolatile memorysuch as flash memory and/or other suitable electronic data storage. Settop box 480 also may support connections with a WLAN via a WLAN networkinterface 496 which may implement the beamforming techniques describedherein.

Referring now to FIG. 2G, the present invention may be embodied in amedia player 500. The present invention may implement either or bothsignal processing and/or control circuits, which are generallyidentified in FIG. 2G at 504, a WLAN interface and/or mass data storageof the media player 500. In some implementations, media player 500includes a display 507 and/or a user input 508 such as a keypad,touchpad and the like. In some implementations, media player 500 mayemploy a graphical user interface (GUI) that typically employs menus,drop down menus, icons and/or a point-and-click interface via display507 and/or user input 508. Media player 500 further includes an audiooutput 509 such as a speaker and/or audio output jack. Signal processingand/or control circuits 504 and/or other circuits (not shown) of mediaplayer 500 may process data, perform coding and/or encryption, performcalculations, format data and/or perform any other media playerfunction.

Media player 500 may communicate with mass data storage 510 that storesdata such as compressed audio and/or video content in a nonvolatilemanner. In some implementations, the compressed audio files includefiles that are compliant with MP3 format or other suitable compressedaudio and/or video formats. The mass data storage may include opticaland/or magnetic storage devices for example hard disk drives HDD and/orDVDs. At least one HDD may have the configuration shown in FIG. 2Aand/or at least one DVD may have the configuration shown in FIG. 2B. TheHDD may be a mini HDD that includes one or more platters having adiameter that is smaller than approximately 1.8″. Media player 500 maybe connected to memory 514 such as RAM, ROM, low latency nonvolatilememory such as flash memory and/or other suitable electronic datastorage. Media player 500 also may support connections with a WLAN via aWLAN network interface 516 which may implement the beamformingtechniques described herein. Still other implementations in addition tothose described above are contemplated.

Referring to FIG. 2H, the present invention may be embodied in a Voiceover Internet Protocol (VoIP) phone 600 that may include one or moreantennas 618, either or both signal processing and/or control circuits,which are generally identified in FIG. 2H at 604, and a wirelessinterface and/or mass data storage of the VoIP phone 600. In someimplementations, VoIP phone 600 includes, in part, a microphone 610, anaudio output 612 such as a speaker and/or audio output jack, a displaymonitor 614, an input device 616 such as a keypad, pointing device,voice actuation and/or other input devices, and a Wireless Fidelity(Wi-Fi) communication module 608. Signal processing and/or controlcircuits 604 and/or other circuits (not shown) in VoIP phone 600 mayprocess data, perform coding and/or encryption, perform calculations,format data and/or perform other VoIP phone functions.

VoIP phone 600 may communicate with mass data storage 602 that storesdata in a nonvolatile manner such as optical and/or magnetic storagedevices, for example hard disk drives HDD and/or DVDs. At least one HDDmay have the configuration shown in FIG. 2A and/or at least one DVD mayhave the configuration shown in FIG. 2B. The HDD may be a mini HDD thatincludes one or more platters having a diameter that is smaller thanapproximately 1.8″. VoIP phone 600 may be connected to memory 606, whichmay be a RAM, ROM, low latency nonvolatile memory such as flash memoryand/or other suitable electronic data storage. VoIP phone 600 isconfigured to establish communications link with a VoIP network (notshown) via Wi-Fi communication module 608 which may implement thebeamforming techniques described herein.

Moreover, while the present invention has been described with referenceto specific examples, which are intended to be illustrative only and notto be limiting of the invention, it will be apparent to those ofordinary skill in the art that changes, additions and/or deletions maybe made to the disclosed embodiments without departing from the spiritand scope of the invention.

What is claimed is:
 1. An apparatus, comprising: a symbol encoder unitconfigured to produce two or more encoded spatial data streams includingat least first and second encoded spatial data streams, wherein anumber, N_(SS), of the encoded spatial data streams is less than anumber, N_(TX), of antennas to be used to transmit the encoded spatialdata streams; and a spatial spreading unit configured to utilize aspatial spreading matrix Q to distribute the two or more encoded spatialdata streams to the antennas, wherein: at least a first portion of thefirst encoded spatial data stream is correlated with a correspondingportion of the second encoded spatial data stream, the spatial spreadingmatrix Q has N_(TX) rows and N_(SS) columns, a ratio of squared norms ofa sum of components of a row, for different rows of the spatialspreading matrix Q, is equal to a first constant sequence, and a ratioof squared norms of a sum of a symbol S₁ to be transmitted, when thesymbol S₁ is equal to 1 or −1, multiplied by each of the components of arow, for different rows of the spatial spreading matrix Q, is equal to asecond constant sequence.
 2. The apparatus of claim 1, wherein the firstconstant sequence is designed based on linear operating regions of poweramplifiers corresponding to the N_(TX) antennas when at least some ofthe power amplifiers have different operating regions.
 3. The apparatusof claim 1, wherein the second constant sequence is designed based onlinear operating regions of power amplifiers corresponding to the N_(TX)antennas when at least some of the power amplifiers have differentoperating regions.
 4. The apparatus of claim 1, wherein the firstconstant sequence is equal to the second constant sequence.
 5. Theapparatus of claim 4, wherein the first constant sequence and the secondconstant sequence are designed based on linear operating regions ofpower amplifiers corresponding to the N_(TX) antennas when at least someof the power amplifiers have different operating regions.
 6. Theapparatus of claim 1, wherein the symbol S₁ is based on a long trainingfield for the two or more encoded spatial data streams.
 7. The apparatusof claim 6, wherein the first portion of the first encoded spatial datastream includes the long training field for the first encoded spatialdata stream.
 8. The apparatus of claim 1, wherein the first portion ofthe first encoded spatial data stream includes a signal field for thefirst encoded spatial data stream.
 9. The apparatus of claim 1, whereinthe first portion of the first encoded spatial data stream includespacket header information for the first encoded spatial data stream. 10.The apparatus of claim 1, wherein the spatial spreading matrix Qsatisfies one or more of the following three constraints: (a) the sum ofsquared norms of the components of a column of the spatial spreadingmatrix Q is the same for each column of the spatial spreading matrix Q,(b) an inner product of any two different columns of the spatialspreading matrix Q is equal to zero for every set of two differentcolumns of the spatial spreading matrix Q, and (c) the ratio of squarednorms of the sums of the components of a row for different rows of thespatial spreading matrix Q is equal to the second constant sequence. 11.The apparatus of claim 10, wherein the spatial spreading matrix Qsatisfies two of the constraints (a), (b), and (c).
 12. The apparatus ofclaim 10, wherein the spatial spreading matrix Q satisfies each of theconstraints (a), (b), and (c).
 13. A method of wirelessly transmittingdata via a plurality of antennas, comprising: encoding the data toproduce two or more encoded spatial data streams for transmission,including at least first and second encoded spatial data streams, viathe antennas, wherein a number, N_(SS), of the encoded spatial datastreams is less than a number, N_(TX), of antennas to be used totransmit the encoded spatial data streams; using a spatial spreadingmatrix Q to distribute the two or more encoded spatial data streams tothe number of the antennas, wherein the spatial spreading matrix Q hascomponents associated with each row of a row dimension having a numberof rows equal to the number of the antennas to be used to transmit theencoded spatial data streams and a column dimension having a number ofcolumns equal to the number of the encoded spatial data streams to betransmitted, and wherein at least a first portion of the first encodedspatial data stream is correlated with a corresponding portion of thesecond encoded spatial data stream, the number of the encoded spatialdata streams is less than the number of the antennas, the spatialspreading matrix Q has N_(TX) rows and N_(SS) columns, a ratio ofsquared norms of a sum of components of a row, for different rows of thespatial spreading matrix Q, is equal to a first constant sequence, and aratio of squared norms of a sum of a symbol S₁ to be transmitted, whenthe symbol S₁ is equal to 1 or −1, multiplied by each of the componentsof a row, for different rows of the spatial spreading matrix Q, is equalto a second constant sequence; and simultaneously transmitting thedistributed encoded spatial data streams via the number of the antennas.14. The method of claim 13, wherein the first constant sequence isdesigned based on linear operating regions of power amplifierscorresponding to the N_(TX) antennas when at least some of the poweramplifiers have different operating regions.
 15. The method of claim 13,wherein the second constant sequence is designed based on linearoperating regions of power amplifiers corresponding to the N_(TX)antennas when at least some of the power amplifiers have differentoperating regions.
 16. The method of claim 13, wherein the firstconstant sequence is equal to the second constant sequence.
 17. Themethod of claim 16, wherein the first constant sequence and the secondconstant sequence are designed based on linear operating regions ofpower amplifiers corresponding to the N_(TX) antennas when at least someof the power amplifiers have different operating regions.
 18. The methodof claim 13, wherein the symbol S₁ is based on a long training field forthe two or more encoded spatial data streams.
 19. The method of claim18, wherein the first portion of the first encoded spatial data streamincludes the long training field for the first encoded spatial datastream.
 20. The method of claim 13, wherein the first portion of thefirst encoded spatial data stream includes a signal field for the firstencoded spatial data stream.
 21. The method of claim 13, wherein thefirst portion of the first encoded spatial data stream includes packetheader information for the first encoded spatial data stream.
 22. Themethod of claim 13, wherein the spatial spreading matrix Q satisfies oneor more of the following three constraints: (a) the sum of squared normsof the components of a column of the spatial spreading matrix Q is thesame for each column of the spatial spreading matrix Q, (b) an innerproduct of any two different columns of the spatial spreading matrix Qis equal to zero for every set of two different columns of the spatialspreading matrix Q, and (c) the ratio of squared norms of the sums ofthe components of a row for different rows of the spatial spreadingmatrix Q is equal to the second constant sequence.
 23. The method ofclaim 22, wherein the spatial spreading matrix Q satisfies two of theconstraints (a), (b), and (c).
 24. The method of claim 22, wherein thespatial spreading matrix Q satisfies each of the constraints (a), (b),and (c).
 25. A tangible, non-transitory computer readable medium ormedia storing instructions that, when executed by one or moreprocessors, cause the one or more processors to: encode data to producetwo or more encoded spatial data streams for transmission, including atleast first and second encoded spatial data streams, via a plurality ofantennas, wherein a number, N_(SS), of the encoded spatial data streamsis less than a number, N_(TX), of antennas to be used to transmit theencoded spatial data streams; and use a spatial spreading matrix Q todistribute the two or more encoded spatial data streams to the number ofthe antennas, wherein the spatial spreading matrix Q has componentsassociated with each row of a row dimension having a number of rowsequal to the number of the antennas to be used to transmit the encodedspatial data streams and a column dimension having a number of columnsequal to the number of the encoded spatial data streams to betransmitted, and wherein at least a first portion of the first encodedspatial data stream is correlated with a corresponding portion of thesecond encoded spatial data stream, the number of the encoded spatialdata streams is less than the number of the antennas, the spatialspreading matrix Q has N_(TX) rows and N_(SS) columns, a ratio ofsquared norms of a sum of components of a row, for different rows of thespatial spreading matrix Q, is equal to a first constant sequence, and aratio of squared norms of a sum of a symbol S₁ to be transmitted, whenthe symbol S₁ is equal to 1 or −1, multiplied by each of the componentsof a row, for different rows of the spatial spreading matrix Q, is equalto a second constant sequence.
 26. The tangible, non-transitory computerreadable medium or media of claim 25, wherein the spatial spreadingmatrix Q satisfies one or more of the following three constraints: (a)the sum of squared norms of the components of a column of the spatialspreading matrix Q is the same for each column of the spatial spreadingmatrix Q, (b) an inner product of any two different columns of thespatial spreading matrix Q is equal to zero for every set of twodifferent columns of the spatial spreading matrix Q, and (c) the ratioof squared norms of the sums of the components of a row for differentrows of the spatial spreading matrix Q is equal to the second constantsequence.
 27. The tangible, non-transitory computer readable medium ormedia of claim 26, wherein the spatial spreading matrix Q satisfies twoof the constraints (a), (b), and (c).
 28. The tangible, non-transitorycomputer readable medium or media of claim 26, wherein the spatialspreading matrix Q satisfies each of the constraints (a), (b), and (c).